Pressurized water nuclear reactors are known, which contain, inside a vessel, assemblies, known as internal equipment, which make it possible, in particular, to provide the support and the screening of the reactor core which consists of fuel assemblies, and which are responsible for guiding the reactor control rods which move vertically inside some of the core assemblies. such internal equipment can be subdivided into lower internal equipment comprising the lower core support plate and the core enclosure, which are suspended inside the vessel, and upper internal equipment which comprises an upper plate and a lower plate, both horizontal, which are connected together by vertical spacers. The lower plate of the upper internal equipment forms the upper core plate.
When the vessel is being fitted out, it is necessary to ensure very good vertical alignment of the upper internal equipment which contains the guide tubes for the control rods and of the lower internal equipment which is responsible for the positioning of the core assemblies. In fact, the reaactor core and the control rod guide tubes are of considerable height, of the order of four meters each. To ensure that the control rods travel under satisfactory conditions inside the guide tubes and in the assemblies situated in the extension of these guide tubes, and to ensure that the control rods fall under gravity into a position of maximum insertion into the core in the event of an emergency shutdown, alignment of the vertical directions in which the control rods travel in the guide tubes and in the assemblies must be ensured within very fine tolerances. These travel directions of the control rods are materialized by guiding members situated in the guide tubes of the upper internal equipment and by the guide tubes which form part of the structure of the assemblies.
The guide tubes in the upper internal equipment consist of an upper part which rests on the upper plate of the upper internal equipment, and a lower part containing the guiding members and arranged between the upper plate of the upper internal equipment and the upper core plate. This lower part of the guide tube rests on the upper core plate and has centering pins intended to cooperate with holes in the upper core plate to ensure the accurate positioning and alignment of the guide tube with the assembly situated vertically below the guide tube.
In the case where new nuclear reactor components are being assembled and adjusted, the alignment-checking operations may be carried out under good conditions, because access is then available to all the parts of the lower and upper internal equipment, which are not irradiated.
In the case of a pressurized water nuclear reactor which has already been in operation, it may be necessary to carry out repairs or replacements of some components of the internal equipment which have sustained some deterioration in use.
Thus, the replacement, during a scheduled shutdown of the power station, of the upper internal equipment assembly of a nuclear reactor which has already been in operation has been envisaged. Such replacement assumes that new, replacement, upper internal equipment is adapted to the irradiated lower internal equipment of the reactor, which is kept in the vessel. This operation must be carried out inside the water-filled vessel, with the irradiated lower internal equipment in place and the core fuel assemblies removed.
Checking of the alignment of the upper internal equipment with the lower internal equipment is very difficult to carry out under these conditions, since direct access to the lower internal equipment kept in the vessel is not available.
Until the present, there was no known device for checking alignment which made it possible to carry out an accurate check of the alignment of new replacement upper internal equipment with irradiated lower internal equipment kept in the vessel.